Shaped glass mirrors can be used in solar concentrating applications, including concentrated photovoltaic (CPV) systems. CPV systems can be designed to use mirrors having any of a variety of shapes, including parabolic-shaped mirrors. CPV systems include a solar receiver upon which concentrated sunlight is directed. Some CPV systems can use a partial parabolic shaped mirror, which is define as a mirror having a curved surface that corresponds to an arc along a parabola. Such an arc need not include the vertex of the parabola.
CPV mirrors have extremely high precision requirements because small deviations from the designed sunlight concentration profile and the desired location of the profile on the solar receiver have a strong negative impact on the power generation of the overall CPV system. Accordingly, the glass mirror bending process should be as free from defect, imperfection, and deformation as possible. Current mirror bending technology relies on sag bending, where a flat glass sheet is heated in a furnace above a rib-based mirror mold. The glass sheet sags when heated under its own weight. The rib-based mold then supports the flexible glass sheet, causing the glass sheet to assume the curved shape of the mold, thereby producing a curved sheet of glass for a mirror.
The rib-based mold is typically constructed of a steel or other similar metal. Such molds, however, produce a mirror with imperfections. Because the glass sheet is a continuous surface and the rib-based mold contacts and supports only portions of the surface, while the remainder of the surface is exposed to the furnace environment, the glass sheet experiences dissimilar rates of thermal expansion and heat transfer between the portions of the sheet contacting the mold and the portions unsupported by the ribs. This arrangement can produce imperfections in the glass sheet. These imperfections later negatively impact the performance of the CPV system.
Additionally, as with any other sheet of material, a sag-bent sheet of glass left to harden in a relatively cooler ambient environment will cool faster at the edges than in the center. This disparity in cooling rates can cause distortions or imperfections in the desired curve shape. Some efforts to mitigate these imperfections in curvature have been proposed, including the use of counterweighting of the edges to induce additional sag, or purposefully-countershaped glass sheets designed to match the curvature of the rest of the glass sheet once sag-bent. Both efforts are imperfect and often function by approximation of the anticipated edge effects, which can be specific to the actual processing conditions and vary between installations or even batches in the same oven. Nonetheless, the impairments in the glass sheet still result in reduced reflection accuracy and reduce the power of a CPV system.